1. Field of the Invention
The present invention relates to a fabrication method of a photomask-blank. In particular, it relates to a fabrication technique of a photomask-blank, which is a base material for a photomask, which is used for fine processing of a semiconductor integrated circuit, a charge-coupled device (CCD), a color filter for a liquid crystal device (LCD), a magnetic head and the like.
2. Description of the Related Art
The packaging density of semiconductor integrated circuits is becoming higher, and the wavelength of exposure light of exposure apparatus used in the lithography technique for fabrication of semiconductor integrated circuits or the like is becoming shorter in order to improve the resolution. According to the lithography roadmap of International Technology Roadmap for Semiconductor (ITRS) updated in 2004, the dominant light sources are shifting from ultraviolet light sources of g-line (wavelength λ=436 nm) or i-line (λ=365 nm) to those of shorter wavelengths, specifically, far-ultraviolet light sources of KrF line (λ=248 nm) or ArF line (λ=193 nm).
Furthermore, in 2007, the half pitch will be 65 nm (hp65), and ArF immersion lithography will be adopted. And in 2010, the half pitch will be 45 nm (hp45), and a combination of F2 or ArF immersion lithography and a resolution enhancement technology (RET) will be adopted.
In this way, it is expected that the demand for the photomask (and the photomask-blank as the base material thereof) in the most advanced technology will be assured until 2010. Furthermore, a possibility is pointed out that the lithography using a photomask will be used for a half pitch of 32 nm (hp32), which is expected to be realized until around 2013, and a half pitch of 22 nm (hp22), which is expected to be realized until around 2016.
According to the Rayleigh's equation for resolution evaluation, the resolvable line width RP and the depth of focus DOF are expressed by the following formulas, where k1 and k2 are proportionality factors.RP=k1λ/NA   (1)DOF=k2λ/NA2   (2)
Thus, in order to make the lithography technique finer, in addition to the shorter wavelength described above, a higher numerical aperture (NA) is required.
According to the “immersion technique” that has recently become a focus of attention as a technique for increasing the NA, the numerical aperture is increased by filling the space between a wafer to be exposed to light and a lens placed closest to the wafer with a liquid having a refractive index (n) higher than that of the environmental atmosphere (gas), thereby magnifying the NA value by a factor of the refractive index (n) of the liquid.
Supposed that the divergence of light flux focused on one point on the wafer to be exposed to light is denoted by ±θ, and the refractive index of the space between the wafer and the lens is n0, the numerical aperture NA is expressed as NA=n0*sinθ. Typically, the space between the wafer and the lens is filled with air (n0=1), so that NA=sinθ. On the other hand, if the space between the wafer to be exposed to light and the lens is filled with a liquid having a refractive index of n, NA=n*sinθ. Thus, the numerical aperture NA is increased, and accordingly the resolvable line width RP is reduced.
To achieve a small resolvable line width RP, as can be seen from the formula (1) described above, it is also effective to reduce the proportionality factor k1. An RET to achieve this may be a “modified illumination” in which the shape of the effective light source is modified from the simple circular shape to another shape or a “multiple exposure” such as FLEX in which the wafer is exposed to light by using a single mask and moving the wafer along the optical axis of the projection optical system.
On the other hand, as can be seen from the formula (2) described above, although reduction in wavelength of the exposure light is effective for reduction in resolvable line width RP, it has a problem that it results in a reduction in depth of focus DOF, which in turn adversely affects the production yield. In other words, reduction in wavelength of the exposure light results in a reduction in factor k and thus is advantageous for transfer of a fine structure. However, reduction in wavelength of the exposure light results also in a reduction in depth of focus DOF and thus has a problem that it causes a focus error to reduce the production yield if the flatness of the photomask is insufficient.
One of methods of solving this problem is a phase-shift method. According to the phase-shift method, a phase-shift mask is used, in which patterns are formed in such a manner that patterns adjacent to each other have phases different by approximately 180 degrees from each other.
That is, since the phase-shift film provided on the phase-shift mask shifts the phase of the exposure light by 180 degrees, the light passing through the region in which the phase-shift film pattern is formed and the light passing through the region in which no phase-shift film is formed have an optical intensity of 0 at the boundary between the regions, so that the resulting optical intensity distribution exhibits an abrupt change at the boundary.
As a result, a high DOF can be achieved, and the image contrast is improved.
The phase-shift mask includes a Levenson type and a half-tone type, for example. In particular, the DOF can be significantly improved by using a half-tone phase-shift mask.
As a half-tone phase-shift mask, a single-layer mask having a relatively simple structure has been proposed, and there have been proposed single-layer phase-shift masks that have a phase-shift film made of molybdenum silicide oxide (MoSiO) and molybdenum silicide oxynitride (MoSiON) (see Japanese Patent Laid-Open No.7-140635 (prior-art literature 1), for example).
According to a method of fabricating such a phase-shift mask, a phase-shift mask blank is patterned by lithography. The lithography method involves applying a resist to a phase-shift mask blank, exposing a desired part of the resist to an electron beam or ultraviolet rays, and then developing the resist to expose the exposed part of the surface of the phase-shift film. Then, the exposed phase-shift film is removed by etching using the patterned resist film as a mask to expose the substrate surface, and then, the resist film is peeled off. In this way, a phase-shift mask is provided.
In the case where a plurality of photomask are used to form a multilayer device, a high alignment precision is needed. The alignment precision is inevitably raised as the pattern becomes finer.
However, if a stress is already accumulated in the thin film formed on the substrate at the stage of the photomask-blank, some of the stress accumulated in the film is released in the course of the pattern writing including the resist application step, the exposure step, the development step, the etching step and the resist peel-off step, thereby causing a distortion in the resulting photomask. Such a distortion reduces the alignment precision of the photomask and causes a defect of the circuit pattern to be written.
The level of distortion depends on the pattern to be written and the magnitude of the stress accumulated in the film, and it is extremely difficult to control or release the distortion during the fabrication process of the photomask.
Of course, such a problem does not arise if each thin film is formed under a condition that the stress in the thin film is substantially 0. However, it is virtually impossible to achieve this condition, because it is extremely difficult to find the fabrication process condition that satisfies both the film deposition condition that assures required properties of a thin film as an optical film and the condition for forming a thin film having a low stress.
Thus, a step of depositing a thin film under a condition that assures required properties of a thin film and a step of reducing the stress in the thin film have to be separately provided.
Generally, in the photomask-blank, a thin film, such as a phase-shift film, is deposited by sputtering. However, a stress occurs in the film during the film deposition process, and the stress causes a distortion of the substrate and a warpage of the photomask-blank. To solve the problem, there has been proposed a technique that irradiates a light-absorbing thin film, such as a phase-shift film, with light of a predetermined energy density from a flash lamp to control the stress in the film, thereby reducing the warpage of the photomask-blank (see Japanese Patent Laid-Open No. 2004-0223 (prior-art literature 2)).
As means of externally imparting energy for reducing the stress in the thin film, a hot plate, a heater, a halogen lamp, an infrared lamp, a furnace, rapid thermal anneal (RTA) and the like are also possible. However, these means have a problem that the substrate itself is damaged because the temperature of the substrate is increased by excessively imparted energy or the productivity is reduced because the processing time is elongated. Thus, the light irradiation by means of a flash lamp described in the prior-art literature 2 is preferred.
According to the technique described in the prior-art literature 2, by irradiating the thin film formed on the photomask-blank with light having an appropriate amount of energy from the flash lamp, the stress in the film on the photomask-blank (in particular, the half-tone phase-shift mask) can be reduced to 0.2 μm or less in terms of warpage 0.1 μm or less under a more appropriate condition). As for the optical property of the half-tone phase-shift film, an in-plane phase difference distribution of 0.63 degrees and an in-plane transmittance distribution of 0.13% are achieved.
However, in the case where the processing using the flash lamp is performed as described in the prior-art literature 2, the flash light irradiation causes generation of particles in the chamber by the mechanism described later, and the particles adhere to the substrate and cannot be removed by the subsequent cleaning. Thus, there is a problem that the number of particle-caused defects of the photomask-blank increases.